The present invention relates to data transmission systems, and more particularly to a digital audio transmission system for transmitting broadcast or production quality audio program material over the intercontinental network of the common carriers such as the telephone companies.
The American Telephone and Telegraph Company Technical Advisory No. 34 establishes an interconnection specification for digital cross-connects. The described network is a digital hierarchy to accommodate sources of digital signals which operate at different bit rates. At any given hierarchy level the different digital signal sources must have certain common characteristics to permit interconnection with the common carrier transmission facilities at that level as well as with multiplexers connecting that level to a higher level. The present defined levels are: DS1 (1.544 Mb/s), DS1C (3.152 Mb/s), DS2 (6.312 Mb/s), DS3 (44.736 Mb/s) and DS4 (274.176 Mb/s). Digital equipment is terminated at a standard level cross-connect (DSX-N), and the interconnection specifications for the cross-connect describe the required characteristics for the digital equipment interconnected by the cross-connect.
The DS1 signal is the lowest level in the digital hierarchy at 1.544 Mb/s, but is sufficient to provide for quality transmission of audio signals The DSX-1 specification requires that the digital signals be bipolar with at least 12.5 percent average ones density and no more than fifteen consecutive zeros. The pulses shall fit within an essentially square wave template with a specified pulse amplitude. One DS1 signal normally contains 24 DS0 signals, where a DS0 signal is one normal telephone call. This telephone audio information is received at the central office on a standard twisted pair cable and sampled at a rate of 8 kHz with a resolution of 8 bits. The 24 DS0 8-bit samples are then time division multiplexed with the addition of one framing bit to form a 193-bit frame. The frame rate is the same as the DS0 sampling rate of 8 kHz, thus leading to a data rate for the DS1 channel of 1.544 Mb/s. A master frame is composed of 193-bit subframes of two types, timing and signaling. The timing frames are so named because the framing bit is used to extract the synchronization information of the master frame at a receiver, and the signaling frames are so named because in these frames DS0 bits may be overridden by telephone company signaling information. These two types of frames alternate.
What is desired is a method for transmitting high precision analog signals, such as studio quality audio signals, over common carrier networks in a manner compatible with the common carrier cross-connect system.